This invention relates to variable power sighting scopes, and in particular to a scope that provides magnification zoom ratios greater than are currently feasible. A rifle scope serves to magnify the target and overlay a visual aiming point, the reticle, on the target. It does this through the use of a series of lenses mounted within a generally tubular body and a combination of mechanisms to adjust lens positions.
The lenses in a scope can be generally divided into three groups: the objective lens group; the erector lens group; and the ocular (or eyepiece) lens group. Depending on the particular design there may be one or more individual lenses in each group within a scope. The erector lens group gets its name from its role in inverting the first intermediate image, which would appear inverted (ie. upside down and flipped horizontally) to a viewer, so that the image will appear “erect” or upright to the viewer. Because of this role, any lens or other optical device that inverts an intermediate image will be termed an erector device or assembly in this application. The objective lens group gathers in the light from the target and projects the target image as a magnified and inverted first intermediate image. The erector lens group projects the first intermediate image to a magnified or reduced, and re-inverted, second intermediate image. The ocular lens group presents the second image to the eye for viewing.
In a typical scope, the erector lens group is part of an erector lens assembly, which includes a guide tube that carries the erector lenses. The guide tube is mounted so that it pivots at one end, with the opposite end being adjustable horizontally and vertically to provide windage and elevation correction.
Variable magnification can be achieved by providing a means of adjusting the position of the erector lenses in relationship to each other within the guide tube. This is typically done through the use of a cam tube which fits closely around the guide tube.
Each erector lens (or erector lens subgroup) is mounted in a lens mount which slides within the guide tube. A guide sleeve attached to the lens mount slides in a straight slot in the body of the guide tube to maintain the orientation of the erector lens. This same guide sleeve also engages an angled, or curving, slot in the cam tube. Turning the cam tube causes the erector lens mount to move along a portion of the length of the guide tube, varying the magnification. Each erector lens or lens group has its own slot in the cam tube, with the configuration of these slots determining the amount and rate of magnification change as the cam tube is turned. Each erector lens mount has a slot follower that fits into the corresponding cam slot.
A typical rifle scope includes a windage and an elevation knob, for adjusting the apparent position of the reticle relative to a zero position, which in windage is ideally the rifle bore sight, but in elevation is, ideally the rifle bore sight plus a slight down angle, to compensate for bullet drop at some nominal range on the order of 100 yards.
Another disadvantage of the windage and elevation knob design, is the typical placement of the knobs in the center of the scope. This limits the placement of a hunter's scope, which is held by a set of mounting rings. It can be desirable to mount a scope fairly far forward to accommodate eye relief (the ideal distance from the eyepiece of the scope to place one's aiming eye). The effort to mount the scope forward on the rifle is sometimes stymied at the point where the windage knob is blocked by the front mounting ring. It would be desirable to have a scope without the structure of the windage and elevation knobs in the middle of the scope, to permit greater freedom of scope placement, when held in place by mounting rings.
The windage and elevation knobs typically change the apparent reticle position by changing the angle of the erector assembly relative to the scope housing. To maximize this affect, it is desirable to have as much air gap between the erector assembly and the scope housing to provide room to change the erector assembly's angular position within the scope housing. This drives the design toward very thin-walled cam tubes. The thin-walled cam tubes are quite delicate and can be easily bent during the manufacturing operation, thereby driving up the reject rate and the costs of manufacturing. The thin-walled cam tubes also decrease the expected useful product life, because each time the rifle is fired the slot follower is jolted violently backwards in its cam slot, potentially damaging the thin-walled tube. This problem is proportional to the caliber of the rifle to which the scope is attached. For .50 caliber rifles, it tends to be impractical to provide a conventional magnifying scope with windage and elevation adjustment due to this problem.
It is also desirable that the cam slots have a constant curvature so that the slot follower can have length along the slot, as opposed to being round. Because, as noted above, rifle scopes suffer repeated recoil shocks, it is desirable to distribute the force from the recoil over a longer slot follower. Changes in cam slot curvature place a limit on the length of a cam slot follower. As the zoom ratio gets larger, it becomes impossible to design a cam slot with constant curvature. Changing slot curvature can also cause a zoom actuator to be more difficult to turn over a portion of its range. Users, however, tend to prefer that an actuator have the same “feel” over its entire range.
In known scope designs the zoom ratio is effectively limited to 4× because increasing the zoom ratio increases the range of spatial relationships between the lenses utilized. This means that different aspects of the lenses' shapes may be critically important at differing zoom settings. In turn, this drives tight tolerances for lens shapes and the means of changing the spatial relationships between the lenses (discussed below), thereby increasing the defect rate, and the cost. Accordingly a need remains for a scope that can provide a zoom ratio greater than 4× without the manufacturing, difficulty of use and lack-of-durability problems arising from the extension of known erector lens assembly designs to provide greater zoom ratios.
Another factor limiting the zoom mechanism is the zoom actuation. Typically, a ring mounted on the scope exterior is attached directly to the cam tube. A slot cut through the scope housing permits the ring to be turned up to a typical maximum of about 180°. But because the direct attachment, the ring cannot be turned further than length of the arc of the slot, which is limited by the need to preserve the structural integrity of the scope housing. It appears that it is known to interpose a gear between a zoom actuation ring and the cam tube, however, thereby obviating the need to have a lengthy slot and permitting cam tube rotation of greater than a one-half rotation. The use of a ring as an actuator is still somewhat awkward, however, as it requires a user to move his hand over the scope to adjust the zoom.